Hydraulic oils and transmission fluids transmit power in hydraulic equipment and are used in power transmission applications. They are incompressible fluids used as the power transmitting media in hydraulic systems. Hydraulic power systems commonly involve a pump (as a power source), a series of tubes or elastomeric hoses for transmitting pressurized fluid, and some type of control (typically a series of valves, actuators, or cylinders). As the power transmission media, hydraulic fluids are indispensable in these systems.

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Hydraulic oils and fluids are used to provide large amounts of power using relatively small tubes and hoses. Typical hydraulic applications include excavator booms, dippers, and buckets; hydraulic brakes; power steering systems; mechanical transmission systems; lifts; and general industrial machinery.

Fluid Characteristics

Hydraulic fluids are typically classified by their function within a system; a single fluid can fulfil more than one of these functions. Five broad function types include: transference of hydraulic energy, lubrication of system pumps, valves, and cylinders; avoiding corrosion; removing impurities and abrasive elements; and dissipating heat

Regardless of primary function, all hydraulic fluids share certain properties which render them suitable for use in hydraulic systems.

Lubricity: The fluid must lubricate each contacted surface or component, especially those which are dynamic or have tight tolerances. Viscosity is a related characteristic which is discussed in detail below.

Chemically and physically stable: The fluid must retain its characteristics throughout large pressure fluctuations as well as during long-term storage. Low volatility and non-toxic makeup are also desirable properties.

Temperature: The oil or fluid must dissipate heat buildup caused by pressure drops, friction, and leakages. In outdoor systems, oils must remain stable in low-temperature environments.

Low foaming properties: The fluid must be able to release gases without foaming. Foaming causes loss of fluid and increases system temperature.

Fire and flash resistance: In critical applications or explosive environments, fluids must have relatively high flash points. Oils intended for these applications should include a non-petroleum makeup or contain large amounts of water.

Other physical characteristics: Low coefficient of expansion and low specific gravity to reduce system volume and weight, respectively.

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